allowed at each observation for the extension to reach its
true value, and the lever must be kept floating between
10. Note the maximum load just before the contraction com-
mences appreciably. If possible note the actual con-
traction at this stage.
11. When contraction commences in real earnest run the
jockey weight back to zero, and then re-advance it gently
so as to only just balance the stress.
12. Note the breaking load, which will be generally much less
than the maximum. Remove the specimen.
13. Measure the final dimensions as regards length over
extreme marks, and at the two inches round the fracture:
also the contracted area.
We are now to state (a) Elastic limit as load and
stress, (b) Maximum load and stress, (c) Extension per
cent in 10 ins. and in the 2 ins. at fracture, (d) Reduction
of area per cent., (e) Modulus of elasticity. The first
step is to carefully plot the stress-strain diagram in two
parts, one shewing the whole life of the bar, and the
other shewing the elastic stage up to yield point with a
large scale of extension.
(a) The Elastic limit may be discovered from the second
diagram, being a little below the yield point, and indicated
by a slight curvature from the straight line. Dividing
the elastic load by the original area of the bar will give
the elastic stress per square inch.
(b) The Maximum load is found from the first diagram as
well as from the figure obtained during experiment, and
the maximum unit stress may be expressed both in terms
"of the original area as well as of the area as contracted
on the measurement of this load.
(c) The Extension per cent, is found both on 10 ins. and
on the 2 ins. of fracture, as
length after fracture — original length